In multicellular organisms, genomic stability is maintained by the combined action of an accurate DNA replication machinery and a complex network of DNA repair pathways. DNA joining is an essential step in DNA replication, in DNA excision repair pathways and in genetic recombination. Three human genes encoding DNA ligases have been identified. Previous biochemical studies have indicated that the product of the LIG1 gene functions in DNA replication. These observations were supported by the abnormal accumulation of Okazaki fragments in cell lines established from a human individual who had inherited mutations in the DNA ligase I gene. The clinical symptoms of this DNA ligase I-deficient individual included growth retardation, lymphoma and immunodeficiency. Since cell lines established from this individual, which have 10-fold less DNA ligase I activity than control cell lines, also exhibit hypersensitivity to DNA damage introduced by ultra violet light, gamma irradiation and alkylating agents, it appears that DNA ligase I may also function in one or more DNA repair pathways. In this project we will elucidate the molecular mechanisms by which DNA ligase I participates in different DNA transactions. We hypothesize that, in vivo, DNA ligase I is recruited to its nicked DNA substrate by specific protein-protein interactions that are mediated by the amino terminal domain of this enzyme. This domain is not required for catalytic activity but is essential for in vivo function. In preliminary studies we have identified an interaction between DNA ligase I and proliferating cell nuclear antigen, a polypeptide that is required for DNA replication and is also involved DNA repair. Furthermore, in collaboration with Dr. Sam Wilson, we have identified an interaction between DNA ligase I and DNA polymerase beta within a multiprotein base excision repair complex. The functional consequences of these interactions will be examined by a combination of in vitro and in vivo studies. Since the N-terminal domain of DNA ligase I is phosphorylated in vivo, we will examine whether post-translational modification of this domain regulates the involvement of DNA ligase I in different DNA transactions. The long term goal of these studies is to understand the molecular mechanisms by which DNA ligase I functions in DNA replication and DNA repair. This will contribute to our knowledge of how the network of DNA metabolic pathways maintains genetic stability and will facilitate studies on the relationship between genetic instability and carcinogenesis.